Valves are common components of almost any piping system. Although they can be constructed in many different configurations and sizes, a typical valve will include a few basic elements. These include: a valve body that houses internal components within an internal cavity; inlet and outlet ports leading to and from the valve body; one or more valve members positioned within the cavity of the valve body with passageways for fluid flow; inlet and outlet seats that seal the contact points between the inlet and outlet ports and the valve member(s); and a valve stem or other structure or mechanism that extends outside the cavity for turning the valve member(s) within the valve body cavity. These components are attached such that fluid entering the valve through the inlet pipe is either allowed to flow through the valve member to the outlet pipe or prevented from such flow based on the orientation of the valve member passageway relative to the valve body.
One specialized valve configuration is the "dual isolation" valve (sometimes also referred to as a "double block-and-bleed" valve), which includes a pair of valve members positioned in series within the valve cavity. Dual isolation valves are typically employed in piping systems in which any leakage through the valve when closed would be extremely detrimental, if not catastrophic. Exemplary uses include situations in which (a) two very volatile materials are separated by the valve, (b) a downstream operator is protected by the valve, (c) cross-contamination of two materials is prevented by the valve, and (d) potable and nonpotable water streams are separated by the valve.
An example of a dual isolation valve is illustrated in U.S. Pat. No. 5,669,415 to Trunk (the Trunk valve). The Trunk valve has two frustoconical plugs, each of which is inverted (i.e., the narrower end of the plug extends upwardly). Of course, the volumetric flow of the valve is dependent on the size and shape of passageways in the plugs; however, the size of the passageways is limited by the size of the plug itself, as sufficient structure must surround the passageway to prevent the plug from fracturing or collapsing during use. Typically, and as illustrated in Trunk, valves having frustoconical plugs include trapezoidal flow passageways in the plugs that match the trapezoidal cross-sectional shape of the plugs in an effort to maximize the cross-sectional surface area of the passageway.
Some dual isolation valves having two frustoconical plugs are configured such that one plug is inverted as described above and the other is not (i.e., the narrower end of the plug extends downwardly). This configuration (exemplified in British Patent No. GB 2 305 713 B) is employed in an attempt to reduce the cavity volume and overall length of the valve; because the plugs are oriented 180 degrees apart about the flow axis, they can be positioned closer together without interfering with one another than is the case for identically oriented plugs.
Unfortunately, the reverse orientation of one plug within a dual isolation valve can create reduction in volumetric flow when trapezoidal cross-section flow passageways are employed. Because the perimeters of the flow passageways are not aligned with each other (as can be the case for dual isolation valves like the Trunk valve), a fluid flowing through the valve is redirected somewhat from its flow path, thereby increasing the turbulence in the flow and reducing flow efficiency. This shortcoming has led to some valves having aligned circular flow passageways; however, such passageways provide less cross-sectional area for flow and, thus, also suffer from reduced volumetric flow.